Abnormal Tides

19 Sep to 12 Oct 2017 Water levels from Bermuda Esso Pier, St. George’s Island. Verified water level observations in green, preliminary water level observations in red. Forecast water level due to the predicted astronomical tide in blue. Water level is with respect to mean higher high water, where positive values indicate normally dry land is inundated. Alternative reference water level marks are on the right.

Over the weekend, the combination of the spring tide and a high amplitude ocean eddy resulted in localized coastal flooding around low-lying areas of Bermuda. Tides were running around 1.5 ft above expected levels which were already higher than normal thanks to a spring tide.

The role of the Astronomical Tides:

The astronomical tides are driven primarily by the gravitational effects of the Moon on the ocean. When the Moon is directly overhead, the water rises in response to the Moon’s gravitational pull. When the Moon is directly underfoot, the water rises again to balance the pull of the Moon on the opposite side of the Earth.

During “Spring” tides, the gravitational pull of the Sun on the oceans acts in the same direction as that from the Moon. This results in higher than normal tides and tidal ranges. Conversely, during “Neap” tides, the gravitational pull of the Sun is acting perpendicular to that of the Moon and lower than normal tides and tidal ranges can be expected. Looking at the blue line in the above figure, higher tides associated with the Spring tide can be seen around the 20th September and again last weekend, while lower tides associated with Neap tide can be seen around the 28th September.

Additionally, the Sun and Moon have to be aligned in space for their gravitational pull to act in the same direction. This manifests as a New Moon when the Moon is between the Sun and the Earth, and a Full Moon when the Earth is in between the Sun and the Moon. Both New and Full moon are associated with Spring tides. The Lunar cycle (including one Full and one New Moon) repeats roughly every 29 days and so you can expect a Spring tide a little more than every fortnight.

Finally, the Moon follows an elliptical orbit around the Earth and so is closer or further away twice per orbit. Every ~7.5 Spring tides, the moon reaches its closest distance to Earth during a New or Full Moon. When the Moon is closer to Earth (perigee), the tides are slightly higher than normal. The opposite is true for when the Moon is furthest from Earth (apogee). Tides during last weekend’s Spring tide were higher than the 20th September’s Spring tide because the Moon was near/at perigee last weekend, and not during the 20th September.

The role of Ocean Eddies:

Sea surface height anomalies showing a positive anomaly greater than 30 cm near Bermuda (circled in black). This was associated with an anticyclonic eddy that intensified as it tracked southwestward toward Bermuda over the last two months.

Ever present in the ocean, eddies can manifest as regions of higher (positive) or lower (negative) sea surface height anomalies. The flow around these sea surface height anomalies is often close to balanced and so they can persist for a long time as they track across the ocean surface. These anomalies are typically small, less than 30 cm.

Typical flow around a positive sea surface height anomaly is clockwise (anticyclonic), and counter-clockwise (cyclonic) for a negative sea surface height anomaly in the northern hemisphere.

Over the weekend, a positive sea surface height anomaly associated with an anticyclonic eddy was tracking near Bermuda with amplitude estimated to be more than 30 cm (1 ft) via satellite measurements. Coinciding with the spring tide and Lunar perigee, this resulted in abnormally high water levels and some coastal inundation.

With sea level rise associated with climate change, it is reasonable to expect this mostly nuisance level of inundating events to occur more frequently as water level anomalies don’t have to be as extreme for flooding to occur.